This application includes disclosure contained in application Ser. No. 09/466,313, filed Dec. 17, 1999, entitled xe2x80x9cBand Pass Filter from Two Notch Filtersxe2x80x9d, assigned to the assignee of this invention. The entire contents of the earlier application is incorporated herein by reference.
This invention relates to band pass filters and, in particular, to improving the separation of parallel band pass filters without changing the filters themselves.
Frequently, a plurality of band pass filters are coupled in parallel and the outputs are summed at an active or passive summation node. As more fully described below, this has the effect of broadening the response curve of each filter in a pair of adjoining filters.
Parallel band pass filters are used in many and diverse applications, such as equalizers, hearing aids, telephones, and other audio and radio frequency applications. To consider but one example, a single telephone may include several sets of filters, e.g. for detecting multiple tone dialing signals, for noise reduction, and for echo cancellation. Devices known as complementary comb filters have been used to eliminate echoes by having the signal to a speaker filtered through the pass bands of a first comb filter, thereby falling within the stop bands of a second, complementary comb filter coupled to a microphone.
Comb filters are used primarily because the xe2x80x9cQxe2x80x9d of most filters is relatively low, less than twenty and typically about ten. One definition of xe2x80x9cQxe2x80x9d is the ratio of the bandwidth at xe2x88x923 dB to the center frequency. The center frequencies in a comb filter are widely spaced, relative to the bandwidth, and band reject or stop band filters are used in between pairs of band pass filters. The xe2x80x9cQxe2x80x9d of a filter is not a very good description of the frequency response of a filter because Q does not describe the shape of the response curve of the filter, particularly the xe2x80x9cskirtsxe2x80x9d of the curve. It is desired that the skirts be as close to vertical as possible, although a vertical skirt can only be approached as a limit.
In view of the foregoing, it is therefore an object of the invention to improve the separation of parallel band pass filters without changing the filters themselves.
Another object of the invention is to reduce the bandwidth of parallel filters without affecting the nominal xe2x80x9cQxe2x80x9d of the filter.
The foregoing objects are achieved in this invention in which the separation of a adjacent band pass filters is improved, without changing the filters, by inverting the output signals from alternate filters and not inverting the remaining output signals. All the output signals are then summed. The result is a deeper notch in the frequency response of adjacent filters.